1. Field of the Invention.
The invention relates to the field of processing audio signals for AM transmitters to compensate for rolloff in typical AM receivers.
2. Prior Art.
While amplitude modulation (AM) broadcast transmitters of current design generally have flat modulation depth as a function of modulation frequency (frequency response), this is not true for typical AM receivers. Most AM receivers, particularly those manufactured between 1970 through 1981, have very poor high frequency response, that is, the high frequencies are rolled-off. The sound from these receivers is dulled and muffled unless high frequency preemphasis is applied to the audio signal prior to its transmission.
In prior art AM transmission systems, high frequency preemphasis or boosting is usually applied with conventional graphic or parametric equalizers having a biquadratic response. These equalizers only approximate the high frequency rolloff characteristics of the typical AM receiver. Indeed, the high frequency rolloff characteristics in these receivers is typically much steeper than can be compensated with equalizers using relatively wideband biquadratic sections as is currently the practice.
As will be seen, the present invention describes a particularly economical circuit for generating an equalization curve which is complementary to typical AM receiver rolloff characteristics. Only a single, non-critical control is used to determine the amount of ultimate high frequency boost provided by the equalizer, thus the invented circuit can be easily used.